How to choose the permeability test method

With the increasing emphasis on the safety of food and drug packaging, the barrier properties of packaging materials that directly affect the shelf life of products have become one of the most concerned performance indicators and promote the promotion of related inspections. At present, the permeability test and moisture permeability test of materials have become more popular. There are several test methods for the detection of each barrier index. These test methods have some differences in the detection principle, which will have a certain impact on the comparability of data between various test methods. We hope to obtain a unified data system to facilitate data comparison of material barrier properties. For the moisture permeability test, the data system of various test methods can be unified by standardization according to the standard requirements. However, in the current breathability test, almost no means is adopted to unify the data system between different test methods. Therefore, for the breathability test method, The choice is very important.

1. Differential pressure and isobaric methods are essentially different in testing

The vacuum method is the most representative test method in the differential pressure method. During the whole vacuum test process, the diffusion of the test gas in the sample is unidirectional, so it can be considered as self-diffusion, using Fick's law. The entire diffusion process can be described. For the isobaric method (the most widely used equal pressure method is the sensor method), there are two kinds of gases in the test environment, and there is a reverse infiltration of carrier gas (nitrogen) from the test lower chamber to the test upper chamber. There are two kinds of substances, which are mutual diffusion. The actual data comparison proves that the isostatic data and the differential pressure data are similar but not identical, indicating that the reverse permeation of nitrogen does affect the oxygen infiltration process. Therefore, there is an essential difference between the isobaric method and the traditional differential pressure method, which are two different types of test methods.

2. Advantages of various breathability test methods

2.1 Vacuum method

Due to the support of the membrane technology theory, the vacuum method has always been used as a basic method in the permeability test, and it is the most versatile test method in the world. The internationally developed vacuum method permeability test standard is also the most, it is often used as the first The law was adopted.
The outstanding advantages of the vacuum method are:

1. For the test gas is not selective, it can detect common inorganic gases such as oxygen, carbon dioxide, nitrogen, and air.

2. The amount of test gas used during the test is small, and there are almost no consumable components in the device structure, so the test cost is very low.

3. The test theory is mature, and the classical permeability theory such as Fick's law is well explained in the academic circles. The essential parameters of the tested material, ie solubility coefficient, diffusion coefficient, and permeability coefficient, can be obtained through experiments. Strong universality, no special agreement, it is the first choice for scientific research institutions, quality inspection agencies.

4. High test efficiency. Since there is a period of vacuum before entering the formal test and the test chamber is required to reach a certain degree of vacuum, if an error occurs in the sample mounting process, it will be discovered during the vacuum, and the operator can replace the sample in time. The test will greatly shorten the invalid test time.

5. Test the gas "pure" in the environment. Since the entire test chamber is evacuated to below 27Pa during the vacuum test, the test chamber is filled with clean test gas, so that the impurity gas (non-test gas) in the entire test environment can be completely ignored. Therefore, the influence of impurity gases on the test can be eliminated.

Of course, because of the 0.1 MPa pressure difference between the two sides of the sample, it is very difficult for the vacuum method to realize the permeability test of the container.

2.2 Sensor method

The test principle of the sensor method is to maintain normal pressure on both sides of the test specimen, so that the pressure on both sides of the specimen is equal, which also lays the foundation for the oxygen permeability test of the container, and can avoid the container due to excessive pressure difference on both sides of the container wall. Burst situation. The shape of the container can be a bottle, a bag, a box, etc. Most of the container packaging on the market today can perform the overall oxygen permeability detection, effectively avoiding the errors caused by the sheet detection estimation.

However, while the variety of test specimens is abundant, the sensor method still needs improvement in terms of test gas types and test costs.

1. Single test gas, high test cost. The use of an oxygen sensor to detect the oxygen transmission rate of a material is the most common application of the sensor method. Of course, if the oxygen sensor is replaced with a carbon dioxide sensor, the carbon dioxide permeability of the material can be detected. Since nitrogen is used as a carrier gas to transport the test gas that has permeated through the sample, the use of a sensor method to detect the nitrogen gas permeability of the material cannot be achieved. However, even if the sensor method is used to detect the oxygen permeability of the material while detecting the carbon dioxide transmission rate, the user also needs to purchase additional carbon dioxide sensors, and the gas sensors used are all consumable type, so the cost of detection is high.

2. The calibration period is short. Since the gas sensor used is a consumable component, the calibration factor obtained by the calibration of the device is not valid for a long period of time. Periodic calibration of the device is required according to requirements, otherwise the accuracy of the test results will be affected.

3. There are many gases in the test environment. The reverse permeation of the carrier gas will affect the forward permeation rate of the test gas (oxygen). This effect cannot be quantified and there are differences due to different materials. Therefore, the sensor method is only applicable to the determination of the individual nature of the sample itself, ie the permeation of oxygen (or carbon dioxide) under prior agreements (eg in the packaging industry in certain regions of the world), rather than for the detection of the permeability parameters of the material. .

3. How to choose the permeability test method

Packaging manufacturers and related research and testing organizations should choose which test method based on the permeability tester? We give some suggestions here.

First of all, the test methods used by the equipment should comply with the relevant national policies and industry standards. For example, in the current national examination of production licenses for plastic packaging, containers, and tools used in foods, the permeability test method for plastics films is specified by the differential pressure method (GB/T 1038). Therefore, the relevant agencies that perform the tests and related film manufacturers are The gas permeability test equipment used should be a differential pressure method. For other agencies engaged in testing and scientific research, national standards should be implemented.

Second, choose according to the application of the market. Both isobaric and differential pressure methods have their own set of data systems. Although many organizations are trying to unify the two data systems, there are no substantive conclusions or results. Therefore, when the user of the equipment purchases the air permeability testing equipment, it is better to select the testing equipment with the same principle as the national organization, so as to avoid the influence of the index of the material due to the different test methods.

Third, for some users with special requirements, it is necessary to fully understand the detection advantages of the differential pressure method and isobaric method. For example, a container manufacturing company purchases equal-pressure equipment that can add accessories to achieve the overall inspection of the container, so that the test data is more practical. If there are many types of test gas, it is necessary to select the differential pressure device. If the test cost is also taken into account, the differential pressure method is a more economical choice.